The present invention generally relates to electrography and more particularly, to a method of developing an electrostatic latent image formed on an electrostatic latent image carrier or support member, by the employment of a magnetic brush developing device.
Conventionally, for a magnetic brush developing device, there has been proposed an arrangement in which a developing sleeve and/or a magnetic roller incorporated in the developing sleeve are driven for rotation so as to develop an electrostatic latent image formed on the surface of an electrostatic latent image support member by a developing material which is transported along the peripheral surface of the developing sleeve in the form of a magnetic brush.
With respect to the prior art developing device as described above, there is known a system in which the magnetic roller is driven for rotation for transporting the developing material over the developing sleeve for improved stirring or agitation of said developing material at a developing region, i.e. at a region where the peripheral surface of the developing sleeve most closely approaches the surface of the electrostatic latent image support member, and another system in which an AC developing bias is applied to a developing electrode (this function is mainly carrried out by the developing sleeve) for the improvement of the developing efficiency.
However, in the known systems as described above, there has been experienced such disadvantages that uneven development as shown in FIG. 1 tends to take place unless the speed of rotation of the magnetic roller is greater than a predetermined speed, and similarly, unless the frequency of the AC developing bias is greater than a predetermined value.
In connection with the above, according to the developing experiments carried out by the present inventors through simultaneous employment of both of the systems as described above, it has been found, contrary to the expectation of the present inventors, that irregular or uneven development including dark and light portions at fixed intervals takes place in the direction of movement of the photosensitive member or photoreceptor (i.e. in the direction indicated by an arrow A in FIG. 1), in spite of the fact that the speed of rotation and the frequency of the AC developing bias were made greater than the predetermined values based on the experiences as described earlier.
As shown in FIG. 2, the developing device employed for the above experiments includes an electrically conductive developing sleeve 1 having an outer diameter of 32 mm, and a magnet roller 2 magnetized with eight poles and rotatably accommodated in said developing sleeve 1, with an AC developing bias source 3 being connected to the developing sleeve 1, while a bristle height adjusting plate 5 provided at a lower side edge of a developer tank 4 defines a bristle height restricting gap of 0.25 mm with the sleeve 1. Adjacent to the developing sleeve 1, there is disposed a photoreceptor drum 6 formed by applying a photosensitive layer 6a of CdS.nCdCO.sub.3 resin photosensitive material onto the peripheral surface of an aluminum drum, driven for rotation in the counterclockwise direction at a circumferential speed of 100 mm/sec, and arranged to have formed, on the peripheral photosensitive surface 6a thereof, an electrostatic latent image after having been charged to -800 V by a corona charging means (not shown). The developing gap between the sleeve 1 and drum 6 is 0.35 mm.
In the developing device as described above, when the development is effected, with the employment of a mono-component magnetic developing material as a developer, and by rotating the magnetic roller 2 in the counterclockwise direction at 1400 r.p.m. and with the impression of an AC developing bias of 2000 V (peak to peak) at 400 Hz on the developing sleeve 1 from the AC developing bias source 3, the uneven development as described with reference to FIG. 1 was produced.
The uneven development is considered to be attributable to the following causes. As shown in FIG. 3, magnetic toner 7 present at the developing region is subjected to a developing force acting in the direction to cause it to adhere to the electrostatic latent image formed on the photosensitive surface 6a of the photoreceptor drum 6 by magnetic action which periodically varies, following rotation of the magnetic roller 2, and also by electrical action periodically varying as the AC developing bias is impressed. The developing force as described above reaches a peak value driving periodical fluctuations of the magnetic action and electrical action as described above, thus resulting in the formation of the uneven development with dark and light portions proportional to the frequency of variation of the developing force peak value due to superposition of the two actions described above.
In connection with the above, the present inventors have carried out investigations, through experiments, into the individual variation of the developing force arising from the magnetic action and the electrical action referred to above, and also into the variation of the developing force due to superposition of the above two actions, with findings as follows.
More specifically, in the case where development is effected by rotating the magnetic roller at 1400 r.p.m., without impression of the AC developing bias, the developing force due to the magnetic action varies at a period t.sub.1 of 5.36 msec. as shown in FIG. 4. When the developing sleeve is driven for rotation at a proper speed in the clockwise direction and simultaneously an AC developing bias of 400 Hz is applied, with the magnetic roller held stationary, the developing force due to electrical action varies at a period t.sub.2 of 2.50 msec. as shown in FIG. 5. During impression of AC develop bias at 400 Hz, with the magnetic roller being driven for rotation at 1400 r.p.m., the peak value of the developing force due to superposition of the two actions (in the form of a beat frequency) varies at a period t.sub.3 of 37.6 msec. as shown in FIG. 6.
The uneven development does not take place in the cases of the periods t.sub.1 and t.sub.2, but is formed in the case of the period t.sub.3 as shown in FIG. 1, and the interval l.sub.1 between the dark portions in the uneven development is 3.76 mm (l.sub.1 =t.sub.3 .multidot.V) i.e. the product of the period t.sub.3 of developing force variation and the circumferential speed V of the photoreceptor drum.
In connection with the above, it has been found in the developing experiments carried out by the present inventors, that whether or not the uneven development as described above takes place depends on the relation between the substantial developing time and the period of the developing force peak value, and more specifically, that if the substantial developing time is longer than the period of the developing force peak value, the uneven development does not take place, while on the contrary, if such substantial developing time is shorter than the period of the developing force peak value, uneven development is undesirably produced.
It should be noted here that the above named "substantial developing time" which is a conception newly defined by the present inventors means the time which is required for one predetermined point on the surface of an electrostatic latent image support member to pass a developing region wherein the electrostatic latent image is substantially developed, and which is peculiar to each magnetic brush developing device. More specifically, the substantial developing time as described above is obtained for a particular magnetic brush developing device by developing an electrostatic latent image while varying the speed of rotation of the magnetic roller without impression of an AC developing bias so as to find a value of the minimum speed of rotation R (in rpm's) at which the formation of the uneven development as shown in FIG. 1 is eliminated, from which the substantial developing time is determined from the following equation: EQU Td=(6/R.multidot.p).times.10.sup.4 ( 3)
where
Td: substantial developing time (msec.) PA1 R: minimum speed of rotation (r.p.m) PA1 p: number of poles of the magnetic roller. PA1 t.sub.1 : period (msec.) of variation of the magnetic action due to rotation of the magnetic roller, PA1 t.sub.2 : period (msec.) of variation of the electrical action due to the impression of the AC developing bias, PA1 n, m: integers having no common division other than 1, PA1 Td: substantial developing time (msec.), thereby the period of the developing force peak value is set so as to be no longer than the substantial developing time.
The substantial developing time as described above varies according to kinds of developing materials employed, the developing gap and moving speed of the electrostatic latent image support member. For example, in the developing device shown in FIG. 2, the minimum number of revolutions R is found to be 300 r.p.m., and when R=300 and p=8 are substituted into the above equation (3), the substantial developing time Td works out to be 25 msec.
Accordingly, in the case where the periods t.sub.1 and t.sub.2 are 5.36 msec. and 2.50 msec., and shorter than the substantial developing time Td at 25 msec. (FIGS. 4 and 5), i.e. when the variations of the magnetic and electrical developing forces are not superposed on each other, the uneven development does not take place, while in the case where the period t.sub.3 is 37.6 msec. (FIG. 6), i.e. when the variations of the magnetic and electrical developing forces are superposed on each other, undesirable uneven development is produced.